Nerve myelination is an essential process in the formation and function of the central nervous system (CNS) and peripheral nervous system (PNS) compartments. The myelin sheath around axons is necessary for the proper conduction of electric impulses along nerves. Loss of myelin occurs in a number of diseases, among which are Multiple Sclerosis (MS) affecting the CNS, Guillain-Barre Syndrome, CIDP and others (see Abramsky and Ovadia, 1997; Trojaborg, 1998,Hartung et al, 1998). While of various etiologies, such as infectious pathogens or autoimmune attacks, demyelinating diseases all cause loss of neurologic function and may lead to paralysis and death. While present therapeutical agents reduce inflammatory attacks in MS and retards disease progression, there is a need to develop therapies that could lead to remyelination and recovery of neurologic function (Abramsky and Ovadia, 1997, Pohlau et al, 1998).
Injury to CNS induced by acute insults including trauma, hypoxia and ischemia can affect both neurons and white matter. Although most attention has been paid to processes leading to neuronal death, increasing evidence suggests that damage to oligodendrocytes, which myelinate axons, is also a specific component of CNS injury. Thus oligodendrocyte pathology was demonstrated at very early phase after-stroke (3 hours) in rats, suggesting that these cells are even more vulnerable to excitotoxic events than neuronal cells (Pantoni et al. 1996). One potential candidate mediating cell death is the marked elevation of glutamate concentration that accompanies many acute CNS injuries (Lipton et al. 1994). Indeed, beside neurons oligodendrocytes were also found to express functional glutamate receptors belonging to the AMPA/kainate subtype. Moreover oligodendrocytes display high vulnerability to glutamate application (McDonald et al. 1998).
Trauma is an injury or damage of the nerve. It may be spinal cord trauma, which is damage to the spinal cord that affects all nervous function that is controlled at and below the level of the injury, including muscle control and sensation, or brain trauma, such as trauma caused by closed head injury.
Cerebral hypoxia is a lack of oxygen specifically to the cerebral hemispheres, and more typically the term is used to refer to a lack of oxygen to the entire brain. Depending on the severity of the hypoxia, symptoms may range from confusion to irreversible brain damage, coma and death.
Stroke is usually caused by ischemia of the brain. It is also called cerebrovascular disease or accident. It is a group of brain disorders involving loss of brain functions that occur when the blood supply to any part of the brain is interrupted. The brain requires about 20% of the circulation of blood in the body. The primary blood supply to the brain is through 2 arteries in the neck (the carotid arteries), which then branch off within the brain to multiple arteries that each supply a specific area of the brain. Even a brief interruption to the blood flow can cause decreases in brain function (neurologic deficit). The symptoms vary with the area of the brain affected and commonly include such problems as changes in vision, speech changes, decreased movement or sensation in a part of the body, or changes in the level of consciousness. If the blood flow is decreased for longer than a few seconds, brain cells in the area are destroyed (infarcted) causing permanent damage to that area of the brain or even death.
A stroke affects about 4 out of 1,000 people. It is the 3rd leading cause of death in most developed countries, including the U.S. The incidence of stroke rises dramatically with age, with the risk doubling with each decade after age 35. About 5% of people over age 65 have had at least one stroke. The disorder occurs in men more often than women.
As mentioned above, a stroke involves loss of brain functions (neurologic deficits) caused by a loss of blood circulation to areas of the brain. The specific neurologic deficits may vary depending on the location, extent of the damage, and cause of the disorder. A stroke may be caused by reduced blood flow (ischemia) that results in deficient blood supply and death of tissues in that area (infarction). Causes of ischemic strokes are blood clots that form in the brain (thrombus) and blood clots or pieces of atherosclerotic plaque or other material that travel to the brain from another location (emboli). Bleeding (hemorrhage) within the brain may cause symptoms that mimic stroke.
The most common cause of a stroke is stroke secondary to atherosclerosis (cerebral thrombosis). Atherosclerosis (“hardening of the arteries”) is a condition in which fatty deposits occur on the inner lining of the arteries, and atherosclerotic plaque (a mass consisting of fatty deposits and blood platelets) develops. The occlusion of the artery develops slowly. Atherosclerotic plaque does not necessarily cause a stroke. There are many small connections between the various brain arteries. If the blood flow gradually decreases, these small connections will increase in size and “by-pass” the obstructed area (collateral circulation). If there is enough collateral circulation, even a totally blocked artery may not cause neurologic deficits. A second safety mechanism within the brain is that the arteries are large enough that 75% of the blood vessel can be occluded, and there will still be adequate blood flow to that area of the brain.
A thrombotic stroke (stroke caused by thrombosis) is most common in elderly people, and often there is underlying atherosclerotic heart disease or diabetes mellitus. This type of stroke may occur at any time, including at rest. The person may or may not lose consciousness.
Strokes caused by embolism (moving blood clot) are most commonly strokes secondary to a cardiogenic embolism, clots that develop because of heart disorders that then travel to the brain. An embolism may also originate in other areas, especially where there is atherosclerotic plaque. The embolus travels through the bloodstream and becomes stuck in a small artery in the brain. This stroke occurs suddenly with immediate maximum neurologic deficit. It is not associated with activity levels and can occur at any time. Arrhythmias of the heart are commonly seen with this disorder and often are the cause of the embolus. Damage to the brain is often more severe than with a stroke caused by cerebral thrombosis. Consciousness may or may not be lost. The probable outcome is worsened if blood vessels damaged by stroke rupture and bleed (hemorrhagic stroke).
Peripheral Neuropathy is a syndrome of sensory loss, muscle weakness and atrophy, decreased deep tendon reflexes, and vasomotor symptoms, alone or in any combination.
The disease may affect a single nerve (mononeuropathy), two or more nerves in separate areas (multiple mononeuropathy), or many nerves simultaneously (polyneuropathy). The axon may be primarily affected (e.g. in diabetes mellitus, Lyme disease, or uremia or with toxic agents) or the myelin sheath or Schwann cell (e.g. in acute or chronic inflammatory polyneuropathy, leukodystrophies, or Guillain-Barré syndrome). Damage to small unmyelinated and myelinated fibers results primarily in loss of temperature and pain sensation; damage to large myelinated fibers results in motor or proprioceptive defects. Some neuropathies (e.g. due to lead toxicity, dapsone use, tick bite, porphyria, or Guillain-Barré syndrome) primarily affect motor fibers; others (e.g. due to dorsal root ganglionitis of cancer, leprosy, AIDS, diabetes mellitus, or chronic pyridoxine intoxication) primarily affect the dorsal root ganglia or sensory fibers, producing sensory symptoms. Occasionally, cranial nerves are also involved (e.g. in Guillain-Barré syndrome, Lyme disease, diabetes mellitus, and diphtheria). Identifying the modalities involved helps determine the cause.
Trauma is the most common cause of a localized injury to a single nerve. Violent muscular activity or forcible overextension of a joint may produce a focal neuropathy, as may repeated small traumas (e.g. tight gripping of small tools, excessive vibration from air hammers). Pressure or entrapment paralysis usually affects superficial nerves (ulnar, radial, peroneal) at bony prominences (e.g. during sound sleep or during anesthesia in thin or cachectic persons and often in alcoholics) or at narrow canals (e.g. in carpal tunnel syndrome). Pressure paralysis may also result from tumors, bony hyperostosis, casts, crutches, or prolonged cramped postures (e.g. in gardening). Hemorrhage into a nerve and exposure to cold or radiation may cause neuropathy. Mononeuropathy may result from direct tumor invasion.
Multiple mononeuropathy is usually secondary to collagen vascular disorders (e.g. polyarteritis nodosa, SLE, Sjögren's syndrome, RA), sarcoidosis, metabolic diseases (e.g. diabetes, amyloidosis), or infectious diseases (e.g. Lyme disease, HIV infection). Microorganisms may cause multiple mononeuropathy by direct invasion of the nerve (e.g. in leprosy).
Polyneuropathy due to acute febrile diseases may result from a toxin (e.g. in diphtheria) or an autoimmune reaction (e.g. in Guillain-Barré syndrome); the polyneuropathy that sometimes follows immunizations is probably also autoimmune.
Toxic agents generally cause polyneuropathy but sometimes mononeuropathy. They include emetine, hexobarbital, barbital, chlorobutanol, sulfonamides, phenytoin, nitrofurantoin, the vinca alkaloids, heavy metals, carbon monoxide, triorthocresyl phosphate, orthodinitrophenol, many solvents, other industrial poisons, and certain AIDS drugs (e.g. zalcitabine, didanosine).
Nutritional deficiencies and metabolic disorders may result in polyneuropathy. B vitamin deficiency is often the cause (e.g. in alcoholism, beriberi, pernicious anemia, isoniazid-induced pyridoxine deficiency, malabsorption syndromes, and hyperemesis gravidarum). Polyneuropathy also occurs in hypothyroidism, porphyria, sarcoidosis, amyloidosis, and uremia. Diabetes mellitus can cause sensorimotor distal polyneuropathy (most common), multiple mononeuropathy, and focal mononeuropathy (e.g. of the oculorrotor or abducens cranial nerves).
Malignancy may cause polyneuropathy via monoclonal gammopathy (multiple myeloma, lymphoma), amyloid invasion, or nutritional deficiencies or as a paraneoplastic syndrome.
Specific mononeuropathies: Single and multiple mononeuropathies are characterized by pain, weakness, and paresthesias in the distribution of the affected nerve. Multiple mononeuropathy is asymmetric; the nerves may be involved all at once or progressively. Extensive involvement of many nerves may simulate a polyneuropathy.
Ulnar nerve palsy is often caused by trauma to the nerve in the ulnar groove of the elbow by repeated leaning on the elbow or by asymmetric bone growth after a childhood fracture (tardy ulnar palsy). The ulnar nerve can also be compressed at the cubital tunnel. Paresthesias and a sensory deficit in the 5th and medial half of the 4th fingers occur; the thumb adductor, 5th finger abductor, and interossei muscles are weak and atrophied. Severe chronic ulnar palsy produces a clawhand deformity. Nerve conduction studies can identify the site of the lesion. Conservative treatment should be attempted before surgical repair is attempted.
The carpal tunnel syndrome results from compression of the median nerve in the volar aspect of the wrist between the transverse superficial carpal ligament and the longitudinal tendons of forearm muscles that flex the hand. It may be unilateral or bilateral. The compression produces paresthesias in the radial-palmar aspect of the hand and pain in the wrist and palm; sometimes pain occurs proximally to the compression site in the forearm and shoulder. Pain may be more severe at night. A sensory deficit in the palmar aspect of the first three fingers may follow; the muscles that control thumb abduction and opposition may become weak and atrophied. This syndrome should be distinguished from C-6 root compression due to cervical radiculopathy.
Peroneal nerve palsy is usually caused by compression of the nerve against the lateral aspect of the fibular neck. It is most common in emaciated bedridden patients and in thin persons who habitually cross their legs. Weakness of foot dorsiflexion and eversion (footdrop) occur. Occasionally, a sensory deficit occurs over the anterolateral aspect of the lower leg and dorsum of the foot or in the web space between the 1st and 2nd metatarsals. Treatment is usually conservative for compressive neuropathies (e.g. avoiding leg crossing). Incomplete neuropathies are usually followed clinically and usually improve spontaneously. If recovery does not occur, surgical exploration may be indicated.
Radial nerve palsy (Saturday night palsy) is caused by compression of the nerve against the humerus, e.g. as the arm is draped over the back of a chair during intoxication or deep sleep. Symptoms include weakness of wrist and finger extensors (wristdrop) and, occasionally, sensory loss over the dorsal aspect of the 1st dorsal interosseous muscle. Treatment is similar to that of compressive peroneal neuropathy.
Polyneuropathies are relatively symmetric, often affecting sensory, motor, and vasomotor fibers simultaneously. They may affect the axon cylinder or the myelin sheath and, in either form, may be acute (e.g. Guillain-Barré syndrome) or chronic (e.g. renal failure).
Polyneuropathy due to metabolic disorders (e.g. diabetes mellitus) or renal failure develops slowly, often over months or years. It frequently begins with sensory abnormalities in the lower extremities that are often more severe distally than proximally. Peripheral tingling, numbness, burning pain, or deficiencies in joint proprioception and vibratory sensation are often prominent. Pain is often worse at night and may be aggravated by touching the affected area or by temperature changes. In severe cases, there are objective signs of sensory loss, typically with stocking-and-glove distribution. Achilles and other deep tendon reflexes are diminished or absent. Painless ulcers on the digits or Charcot's joints may develop when sensory loss is profound. Sensory or proprioceptive deficits may lead to gait abnormalities. Motor involvement results in distal muscle weakness and atrophy. The autonomic nervous system may be additionally or selectively involved, leading to nocturnal diarrhea, urinary and fecal incontinence, impotence, or postural hypotension. Vasomotor symptoms vary. The skin may be paler and drier than normal, sometimes with dusky discoloration; sweating may be excessive. Trophic changes (smooth and shiny skin, pitted or ridged nails, osteoporosis) are common in severe, prolonged cases.
Nutritional polyneuropathy is common among alcoholics and the malnourished. A primary axonopathy may lead to secondary demyelination and axonal destruction in the longest and largest nerves. Whether the cause is deficiency of thiamine or another vitamin (e.g. pyridoxine, pantothenic acid, folic acid) is unclear. Neuropathy due to pyridoxine deficiency usually occurs only in persons taking isoniazid for TB; infants who are deficient or dependent on pyridoxine may have convulsions. Wasting and symmetric weakness of the distal extremities is usually insidious but can progress rapidly, sometimes accompanied by sensory loss, paresthesias, and pain. Aching, cramping, coldness, burning, and numbness in the calves and feet may be worsened by touch. Multiple vitamins may be given when etiology is obscure, but they have no proven benefit.
Uncommonly, an exclusively sensory polyneuropathy begins with peripheral pains and paresthesias and progresses centrally to a loss of all forms of sensation. It occurs as a remote effect of carcinoma (especially bronchogenic), after excessive pyridoxine ingestion (>0.5 g/day), and in amyloidosis, hypothyroidism, myeloma, and uremia. The pyridoxine-induced neuropathy resolves when pyridoxine is discontinued.
Hereditary neuropathies are classified as sensorimotor neuropathies or sensory neuropathies. Charcot-Marie-Tooth disease is the most common hereditary sensorimotor neuropathy. Less common sensorimotor neuropathies begin at birth and result in greater disability. In sensory neuropathies, which are rare, loss of distal pain and temperature sensation is more prominent than loss of vibratory and position sense. The main problem is pedal mutilation due to pain insensitivity, with frequent infections and osteomyelitis.
Hereditary motor and sensory neuropathy types I and II (Charcot-Marie-Tooth disease, peroneal muscular atrophy) is a relatively common, usually autosomal dominant disorder characterized by weakness and atrophy, primarily in peroneal and distal leg muscles. Patients may also have other degenerative diseases (e.g. Friedreich's ataxia) or a family history of them. Patients with type I present in middle childhood with footdrop and slowly progressive distal muscle atrophy, producing “stork legs.” Intrinsic muscle wasting in the hands begins later. Vibration, pain, and temperature sensation decreases in a stocking-glove pattern. Deep tendon reflexes are absent. High pedal arches or hammer toes may be the only signs in less affected family members who carry the disease. Nerve conduction velocities are slow, and distal latencies prolonged. Segmental demyelination and remyelination occur. Enlarged peripheral nerves may be palpated. The disease progresses slowly and does not affect life span. Type II disease evolves more slowly, with weakness usually developing later in life. Patients have relatively normal nerve conduction velocities but low amplitude evoked potentials. Biopsies show wallerian degeneration.
Hereditary motor and sensory neuropathy type III (hypertrophic interstitial neuropathy, Dejerine-Sottas disease), a rare autosomal recessive disorder, begins in childhood with progressive weakness and sensory loss and absent deep tendon reflexes. Initially, it resembles Charcot-Marie-Tooth disease, but motor weakness progresses at a faster rate. Demyelination and remyelination occur, producing enlarged peripheral nerves and onion bulbs seen on nerve biopsy.
The characteristic distribution of motor weakness, foot deformities, family history, and electrophysiologic abnormalities confirm the diagnosis. Genetic analysis is available, but no specific treatment. Vocational counseling to prepare young patients for disease progression may be useful. Bracing helps correct footdrop; orthopedic surgery to stabilize the foot may help.
Neurodegenerative diseases comprise, among others, Alzheimer's disease, Parkinson's disease, Huntington's disease and Amyotrophic Lateral Sclerosis (ALS).
Alzheimer's disease is a disorder involving deterioration in mental functions resulting from changes in brain tissue. This includes shrinking of brain tissues, not caused by disorders of the blood vessels, primary degenerative dementia and diffuse brain atrophy. Alzheimer's disease is also called senile dementia/Alzheimer's type (SDAT). It is the most common cause of intellectual decline with aging. The incidence is approximately 9 out of 10,000 people. This disorder affects women slightly more often than men and occurs primarily in older individuals.
The cause is unknown. The neurochemical factors which may participate in generation of the disease include lack of the substances used by the nerve cells to transmit nerve impulses (neurotransmitters), including acetylcholine, somatostatin, substance P, and norepinephrine. Environmental factors include exposure to aluminum, manganese, and other substances. The infectious factors include prion (virus-like organisms) infections that affect the brain and spinal cord (central nervous system). In some families (representing 5 to 10% of cases) there is an inherited predisposition to development of the disorder, but this does not follow strict (Mendelian) patterns of inheritance. The diagnosis is usually made by ruling out other causes of dementia.
Researchers have found that in families that have multiple members with Alzheimer's, there is a particular gene variation which is common to all of those with the disease. The gene, which produces a substance called apolipoprotein E4, is not said to cause the disease, it's presence simply increases the chances that the disease may eventually occur. There are many people who have the E4 gene and never become afflicted with Alzheimer's.
The onset is characterized by impaired memory, with progressive loss of intellectual function. There may be mood changes, changes in language capability, changes in gait, and other changes as the disorder progresses. There is a decrease in the size (atrophy) of the tissues of the brain, enlargement of the ventricles (the spaces within the brain), and deposits within the tissues of the brain.
Parkinsons's disease is a disorder of the brain characterized by shaking and difficulty with walking, movement, and coordination. The disease is associated with damage to a part of the brain that controls muscle movement. It is also called paralysis agitans or shaking palsy.
The disease affects approximately 2 out of 1,000 people, and most often develops after age 50. It affects both men and women and is one of the most common neurologic disorders of the elderly. The term “parkinsonism” refers to any condition that involves a combination of the types of changes in movement seen in Parkinson's disease, which happens to be the most common condition causing this group of symptoms. Parkinsonism may be caused by other disorders or by external factors (secondary parkinsonism).
Parkinson's disease is caused by progressive deterioration of the nerve cells of the part of the brain that controls muscle movement (the basal ganglia and the extrapyramidal area). Dopamine, which is one of the substances used by cells to transmit impulses (transmitters), is normally produced in this area. Deterioration of this area of the brain reduces the amount of dopamine available to the body. Insufficient dopamine disturbs the balance between dopamine and other transmitters, such as acetylcholine. Without dopamine, the nerve cells cannot properly transmit messages, and this results in the loss of muscle function. The exact reason that the cells of the brain deteriorate is unknown. The disorder may affect one or both sides of the body, with varying degrees of loss of function.
In addition to the loss of muscle control, some people with Parkinson's disease become severely depressed. Although early loss of mental capacities is uncommon, with severe Parkinson's the person may exhibit overall mental deterioration (including dementia, hallucinations, and so on). Dementia can also be a side effect of some of the medications used to treat the disorder.
Huntington's Disease is an inherited, autosomal dominant neurologic disease. It is uncommon, affecting approximately 1 in 10000 individuals (Breighton and Hayden 1981). The disease does not usually become clinically apparent until the fifth decade of life, and results in psychiatric disturbance, involuntary movement disorder, and cognitive decline associated with inexorable progression to death, typically 17 years following onset.
The gene responsible for Huntington's disease is called huntingtin. It is located on chromosome 4p, presenting an effective means of preclinical and antenatal diagnosis. The genetic abnormality consists in an excess number of tandemly repeated CAG nucleotide sequences.
The increase in size of the CAG repeat in persons with Huntington's disease shows a highly significant correlation with age of onset of clinical features. This association is particularly striking for persons with juvenile onset Huntington's disease who have very significant expansion, usually beyond 50 repeats. The CAG repeat length in Huntington's disease families does exhibit some instability that is particularly marked when children inherit the huntingtin gene from affected fathers.
In HD, it is not known how this widely, expressed gene, results in selective neuronal death. Further, sequence analysis revealed no obvious homology to other known genes and no structural motifs or functional domains were identified which clearly provide insights into its function. In particular, the question of how these widely expressed genes cause selective neuronal death remains unanswered.
Amyptrophic Lateral Sclerosis, ALS, is a disorder causing progressive loss of nervous control of voluntary muscles because of destruction of nerve cells in the brain and spinal cord. Amyotrophic Lateral Sclerosis, also called Lou Gehrig's disease, is a disorder involving loss of the use and control of muscles. The nerves controlling these muscles shrink and disappear, which results in loss of muscle tissue due to the lack of nervous stimulation. Muscle strength and coordination decreases, beginning with the voluntary muscles (those under conscious control, such as the muscles of the arms and legs). The extent of loss of muscle control continues to progress, and more and more muscle groups become involved. There may be a loss of nervous stimulation to semi-voluntary muscles, such as the muscles that control breathing and swallowing. There is no effect on ability to think or reason. The cause is unknown.
ALS affects approximately 1 out of 100,000 people. It appears in some cases to run in families. The disorder affects men more often than women. Symptoms usually do not develop until adulthood, often not until after age 50.
Traumatic nerve injury may concern the CNS or the PNS. Traumatic brain injury (TBI), also simply called head injury or closed head injury (CHI), refers to an injury where there is damage to the brain because of an external blow to the head. It mostly happens during car or bicycle accidents, but may also occur as the result of near drowning, heart attack, stroke and infections. This type of traumatic brain injury would usually result due to the lack of oxygen or blood supply to the brain, and therefore can be referred to as an “anoxic injury”.
Brain injury or closed head injury occurs when there is a blow to the head as in a motor vehicle accident or a fall. In this case, the skull hits a stationary object and the brain, which is inside the skull, turns and twists on its axis (the brain stem), causing localised or widespread damage. Also, the brain, a soft mass surrounded by fluid that allows it to “float,” may rebound against the skull resulting in further damage.
There may be a period of unconsciousness immediately following the trauma, which may last minutes, weeks or months. Due to the twisting and rebounding, the traumatically brain injured patient usually receives damage or bruising to many parts of the brain. This is called diffuse damage, or “non-missile injury” to the brain. The types of brain damages occurring in non-missile injuries may be classified as either primary or secondary.
Primary brain damage occurs at the time of injury, mainly at the sites of impact, in particular when a skull fraction is present. Large contusions may be associated with an intracerebral hemorrhage, or accompanied by cortical lacerations. Diffuse axonal injuries occur as a result of shearing and tensile strains of neuronal processes produced by rotational movements of the brain within the skull. There may be small hemorrhagic lesions or diffuse damage to axons, which can only be detected microscopically.
Secondary brain damage occurs as a result of complications developing after the moment of injury. They include intracranial hemorrhage, traumatic damage to extracerebral arteries, intracranial herniation, hypoxic brain damage or meningitis.
An open head injury is a visible assault to the head and may result from a gunshot wound, an accident or an object going through the skull into the brain (“missile injury to the brain”). This type of head injury is more likely to damage a specific area of the brain.
So called mild brain injury may occur with no loss of consciousness and possibly only a dazed feeling or confused state lasting a short time. Although medical care administered may be minimal, persons with brain injury without coma may experience symptoms and impairments similar to those suffered by the survivor of a coma injury.
In response to the trauma, changes occur in the brain which require monitoring to prevent further damage. The brain's size frequently increases after a severe head injury. This is called brain swelling and occurs when there is an increase in the amount of blood to the brain. Later in the illness water may collect in the brain which is called brain edema. Both brain swelling and brain edema result in excessive pressure in the brain called intracranial pressure (“ICP”).
Spinal cord injuries account for the majority of hospital admissions for paraplegia and tetraplegia. Over 80% occur as a result of road accidents. Two main groups of injury are recognised clinically: open injuries and closed injuries.
Open injuries cause direct trauma of the spinal cord and nerve roots. Perforating injuries can cause extensive disruption and hemorrhage. Closed injuries account for most spinal injuries and are usually associated with a fracture/dislocation of the spinal column, which is usually demonstrable radiologically. Damage to the cord depends on the extent of the bony injuries and can be considered in two main stages: Primary damage, which are contusions, nerve fibre transections and hemorrhagic necrosis, and secondary damage, which are extradural heamatoma, infarction, infection and edema.
Late effects of cord damage include: ascending and descending anterograde degeneration of damaged nerve fibers, post-traumatic syringomelyia, and systemic effects of paraplegia, such as urinary tract and chest infections, pressure sores and muscle wasting.
Neurologic disorders may further be due to congenital metabolic disorders. Myelin sheaths, which cover many nerve fibers, are composed of lipoprotein layers formed in early life. Myelin formed by the oligodendroglia in the CNS differs chemically and immunologically from that formed by the Schwann cells peripherally, but both types have the same function: to promote transmission of a neural impulse along an axon.
Many congenital metabolic disorders (e.g. phenylketonuria and other aminoacidurias; Tay-Sachs, Niemann-Pick, and Gaucher's diseases; Hurler's syndrome; Krabbe's disease and other leukodystrophies) affect the developing myelin sheath, mainly in the CNS. Unless the biochemical defect can be corrected or compensated for, permanent, often widespread, neurologic deficits result.
For instance, Krabbe disease or globoid cell leukodystrophy is a disorder involving the white matter of the peripheral and central nervous systems. Mutations in the gene for the lysosomal enzyme galactocerebrosidase (GALC) result in low enzymatic activity and decreased ability to degrade galactolipids found almost exclusively in myelin. Continued myelination and/or remyelination in patients requires functional endogenous oligodendrocytes or transplantation of normal oligodendrocytes or stem cells that can differentiate into oligodendrocytes, in order to provide for sufficient GALC expression (Wenger et al., 2000).
Neurofibromatosis 1 (NF1) is a common autosomal disorder with a wide range of neurologic manifestations.
Multiple system atrophy is a sporadic, adult-onset neurodegenerative disease of unknown etiology. The condition may be unique among neurodegenerative diseases by the prominent, if not primary, role played by the oligodendroglial cell in the pathogenetic process. The major difference to Parkinson's disease is that MSA patients do not respond to L-dopa treatment.
Demyelination in later life is a feature of many neurologic disorders; it can result from damage to nerves or myelin due to local injury, ischemia, toxic agents, or metabolic disorders. There is also evidence that demyelination may contribute to schizophrenia. Extensive myelin loss is usually followed by axonal degeneration and often by cell body degeneration, both of which may be irreversible. However, remyelination occurs in many instances, and repair, regeneration, and complete, recovery of neural function can be rapid. Central demyelination (ie, of the spinal cord, brain, or optic nerves) is the predominant finding in the primary demyelinating diseases, whose etiology is unknown. The most well known is MS.
Acute disseminated encephalomyelitis, postinfectious encephalomyelitis is characterized by perivascular CNS demyelination, which can occur spontaneously but usually follows a viral infection or viral vaccination (or, very rarely, bacterial vaccination), suggesting an immunologic cause. Acute inflammatory peripheral neuropathies that follow a viral vaccination or the Guillain-Barré syndrome are similar demyelinating disorders with the same presumed immunopathogenesis, but they affect only peripheral structures.
Metachromatic leukodystrophy is another demyelinating disease. Adrenoleukodystrophy and adrenomyeloneuropathy are rare X-linked recessive metabolic disorders characterized by adrenal gland dysfunction and widespread demyelination of the nervous system. Adrenoleukodystrophy occurs in young boys; adrenomyeloneuropathy, in adolescents. Mental deterioration, spasticity, and blindness may occur. Adrenoleukodystrophy is invariably fatal. Dietary and immunomodulatory treatments are under study.
Leber's hereditary optic atrophy and related mitochondrial disorders are characterized primarily by bilateral loss of central vision, usually affecting young men in their late teens or early twenties. Leber's hereditary optic atrophy can resemble the optic neuritis in MS. Mutations in the maternally inherited mitochondrial DNA have been identified.
HTLV-associated myelopathy, a slowly progressive spinal cord disease associated with infection by the human T-cell lymphotrophic virus, is characterized by spastic weakness of both legs.
Further neurologic disorders comprise neuropathies with abnormal myelination, an overview of which is given below.
Immune: Acute, Guillain Barré, Chronic, Chronic Immune Demyelinating Polyneuropathy (CIDP), Multifocal CIDP, Multifocal Motor Neuropathy (MMN), Anti-MAG Syndrome, GALOP Syndrome, Anti-Sulfatide Antibody Syndrome (with serum M-protein), Anti-GM2 antibody syndrome, POEMS Syndrome, Polyneuropathy Organomegaly, Endocrinopathy or Edema, M-protein, Skin changes, Perineuritis, IgM anti-GD1b antibody syndrome (occasional).
Toxins: Diphtheria, Buckthorn, Hexachlorophene, Sodium Cyanate, Tellurium.
Drugs: Predominantly demyelinating: Chloroquine, FK506 (Tacrolimus), Perhexiline, Procainamide, Zimeldine; Mixed demyelinating & axonal: Amiodarone, Eosinophilia-Myalgia syndrome, Gold, Suramin, Taxol.
Hereditary: Carbohydrate-deficient glycoprotein, Cataracts & Facial dysmorphism, Cockayne's syndrome, Congenital hypomyelinating, Congenital muscular dystrophy: Merosin deficient, Farber's disease (Lipogranulomatosis), HMSN & CMT, Dominant: IA, IB, III, HNPP, EGR2, Thermosensitive, Recessive: III (Dejerine-Sottas); 4A; 4B; 4B2; 4C; 4D (LOM); 4E; 4F; HMSN-R; CNS, X-linked: IX, Krabbe, Marinesco-Sjögren, Metachromatic Leukodystrophy, Niemann-Pick, Pelizaeus-Merzbacher (PLP), Refsum, Prion protein (PrP27-30): Glu200Lys mutation, Creutzfeld-Jakob disease, Mouse model: Prion over expression, Salla disease, SOX10, Tenascin-XA, Uneven packing of peripheral myelin sheaths, Ehlers-Danlos phenotype.
Metabolic (unusual): Diabetes (due to concurrent CIDP), Hypothyroidism, Hepatic disorders.
Mitochondrial: MNGIE Syndrome, Myopathy & external ophthalmoplegia, neuropathy, Gastro-Intestinal Encephalopathy, NARP Syndrome, Neuropathy, Ataxia, Retinitis, Pigmentosa.
Infections: Creutzfeld-Jakob disease, Diphtheria, HIV: Associated CIDP, Leprosy: Lepromatous; Mixed axonal-demyelinating; Colonized Schwan cells, Variant Creutzfeld-Jakob disease.
Further details can be taken from the following internet-site: www.neuro.wustl.edu/neuromuscular/nother/myelin.html.
Multiple Sclerosis (MS) is an inflammatory demyelinating disease of the central nervous system (CNS) that takes a relapsing-remitting or a progressive course. MS is not the only demyelinating disease. Its counterpart in the peripheral nervous system (PNS) is chronic inflammatory demyelinating polyradiculoneuropathy (CIDP). In addition, there are acute, monophasic disorders, such as the inflammatory demyelinating polyradiculoneuropathy termed Guillain-Barré syndrome (GBS) in the PNS, and acute disseminated encephalomyelitis (ADEM) in the CNS. Both MS and GBS are heterogeneous syndromes. In MS different exogenous assaults together with genetic factors can result in a disease course that finally fulfils the diagnostic criteria. In both diseases, axonal damage can add to a primarily demyelinating lesion and cause permanent neurologic deficits.
MS is the most common of the above demyelinating diseases. It is characterized as an autoimmune disorder, in which leukocytes of the immune system launch an attack on the white matter of the central nervous system (CNS). The grey matter may also be involved. Although the precise etiology of MS is not known, contributing factors may include genetic, bacterial and viral infection. In its classic manifestation (85% of all cases), it is characterized by alternating relapsing/remitting phases, which correspond to episodes of neurologic dysfunction lasting several weeks followed by substantial or complete recovery (Noseworthy, 1999). Periods of remission grow shorter over time. Many patients then enter a final disease phase characterized by gradual loss of neurologic function with partial or no recovery. This is termed secondary progressive MS. A small proportion (˜15% of all MS patients) suffers a gradual and uninterrupted decline in neurologic function following onset of the disease (primary progressive MS). There is currently no clear curative treatment for the severest forms of MS, which are generally fatal.
The basic hallmark of MS is the demyelinated plaque with reactive glial scar formation, seen in the white matter tracts of the brain and spinal cord. Demyelination is linked to functional reduction or blockage in neural impulse conduction. Axonal transection and death is also observed in MS patients (Bjartmar et al., 1999). Pathological studies show the majority of involvement limited to the optic nerves, periventricular white matter, brain stem and spinal cord (Storch et al., 1998). The effects of these CNS deficiencies include the acute symptoms of diplopia, numbness and unsteady gait, as well as chronic symptoms such as spastic paraparesis and incontinence.
Molecular mechanisms underlying MS pathogenesis appear to stem from genetic and environmental factors, including viral and bacterial infections. These mechanisms promote increased migration of T lymphocytes and macrophages across the blood-brain barrier and into CNS tissue.
Demyelination is caused by attacks on myelin by activated macrophages and microglia, as well as damage to myelinating cells stemming from Fas-ligand signaling and complement- or antibody-mediated cytotoxicity. Therefore, demyelination occurs through both a direct attack on the myelin sheaths as well as elimination of the cells that produce and maintain myelin.
Genetic and environmental elements lead to an increased influx of inflammatory cells across the blood-brain barrier. This results in the increased migration of autoreactive T lymphocytes and macrophages into CNS tissue. Cytokine secretion by T cells activates antigen-presenting cells (APCs). When autoreactive T cells in the context of MHC class II molecules on APCs encounter putative ‘MS antigens’, often protein constituents of the myelin sheath, they may become activated. Several subsequent mechanisms can then act to damage oligodendrocytes and myelin. Complement- and antibody-mediated cytotoxicity may cause the majority of damage in some patients, while Fas-ligand signaling, and release of pro-inflammatory cytokines like TNF-α by CD4+ T cells may attack white matter in others. Activated macrophages may also play a role through enhanced phagocytosis and factor secretion. This causes widespread demyelination and, subsequent loss of conduction efficiency among the axons of the CNS. Subsequent repair mechanisms can, however, give rise to remyelination once the inflammatory process is resolved. The remyelinated axons of MS patients are recognized pathologically by the thin appearance of the sheaths around the remyelinated axons. Additional sodium channels are often found inserted into the demyelinated axonal membrane, compensating for the loss of conduction efficiency. Oligodendroglial precursors may enhance remyelination in MS lesions.
The oligodendrocyte performs a multitude of functions related to its production and maintenance of the myelin sheath. This provides insulation, support and conductance enhancement for the axons of multiple neurons. A single oligodendrocyte may myelinate up to 50 different axons. Myelination is restricted only to certain, large diameter axons; dendrites and other cell processes, such as those of astrocytes, remain unmyelinated. Axons appear to exert control over the number of myelinating oligodendrocytes, since axonal transection in the paradigm of the rat optic nerve inhibits myelin renewal and oligodendrocyte precursor production (reviewed in Barres and Raff, 1999). Oligodendrocyte proliferation and migration may be stimulated by factors released from axons during development. In this manner, the numbers of oligodendrocytes and axons are carefully matched within the CNS.
Oligodendrocytes, the perineuronal support cells of the CNS, myelinate axonal tracts and serve to enhance impulse transduction. They play roles in axonal survival and function. Note that, as shown in this diagram, an oligodendrocyte extends only one process to each axon it myelinates.
The multilamellar myelin sheath is a specialized domain of the glial cell plasma membrane, rich in lipid and low in protein. It serves to support axons and improve the efficiency of electrical signal conduction in the CNS by preventing the charge from bleeding off into the surrounding tissue. The nodes of Ranvier are the sites in the sheath along the axon where saltatory conductance occurs.
In the adult brain, oligodendrocytes develop from as yet poorly defined precursor cells in the subventricular zone of the brain and spinal cord (Nait-Oumesmar et al., 1999). These precursors are proliferative and express myelin transcripts and proteins, first emerging in the ventral region of the embryonic spinal cord several weeks before myelination (Hajihosseini et al., 1996). The process of myelination occurs in the post-natal brain. During post-natal development, these precursors migrate to the neuron tracts that are to be myelinated.
Oligodendrocytes mature from their precursor cells in a defined and specific manner (reviewed e.g. in Rogister et al., 1999). Oligodendrocyte development follows a defined pathway at which each stage is demarcated by several cell-specific markers: endothelial neural cell adhesion molecule (E-NCAM), vimentin, A2B5, the POU transcription factor Tst-1/Oct6/SCIP, pre-oligodendroblast antigen (POA), galactocerebroside (GalC), O1, O4, and the myelin-specific; proteins PLP, MBP, and MOG. Neural stem cells give rise to bipolar pre-GD3+cells, which become O2A precursors. These cells can give rise to either oligodendrocytes or type 2 astrocytes. Progression continues through the pre-oligodendroglial and pre-GalC+stages, before actual differentiation into oligodendrocytes. The end stages of the oligodendroglial lineage are defined by these cells' inability to proliferate. Mature oligodendrocytes express the cell-specific markers GalC and sulfatide (SUL), in addition to expressing myelin-specific proteins.
Oligodendrocytes therefore differentiate from mitotically active, migratory precursor cells. Once these cells have become post-mitotic, they transcribe and translate genes encoding myelin-specific proteins. The elaboration of the myelin sheath wrapping the axon is brought about by direct contact between the processes of the mature oligodendrocyte and the axon itself. CNS axon ensheathment is completed by compaction of the myelin sheath, which in its final form resembles a liquid crystal containing macromolecules in complex formation (Scherer, 1997). Promotion of myelination; demands consideration of the precise stoichiometric relationship between the individual structural proteins of the myelin sheath, since increasing or decreasing the amount of one component could result in perturbation of the entire sheath structure.
The inability of oligodendrocytes to sustain repair of demyelinated axons contributes to the cumulative neurologic dysfunction characterizing MS. Promotion of remyelination in MS patients could protect axonal loss and thus limit the progression in disability associated with the death of axons in the CNS.
The demyelinating phenotype of MS led to extensive studies on the nature of the active MS lesion. Naked axons and the absence of myelinating oligodendrocytes indicated the disruption of normal myelin and aberrations in the remyelinating process associated with MS. About 40% of MS lesions were shown to exhibit evidence of abortive remyelination, especially in the early phases of the disease (Prineas et al., 1993). This presents the realistic prospect that developing strategies for promoting myelin repair could prevent permanent nervous system damage. Success probability is particularly high in younger CNS lesions, where early remyelination has already been shown to take place. However, the myelinating or remyelinating oligodendrocyte is a cell under extreme metabolic stress, which under pressure of even minor additional insults can be irreversibly damaged (Scolding and Lassmann, 1996). This decreases the probability of spontaneous repair in an active MS lesion, where inflammation and other detriments pose obstacles to remyelination. Strategies promoting myelin repair may thus stack the odds further in favor of remyelination and axonal protection in active MS lesions.
The adult human CNS has been shown to contain oligodendrocyte precursor cells that are capable of proliferating, and which could mature into myelinating oligodendrocytes. In addition, it appears that the endogenous oligodendrocyte precursor populations adjacent to MS lesions are depleted during the chronic phases of the disease, due to inhibition of these precursors' ability to proliferate and differentiate (Wolswijk, 1998). Such precursor cells are generally quiescent in the environment of a chronic MS lesion, preventing them from actively contributing to remyelination. The situation in chronic MS lesions could therefore involve factors that hamper oligodendroglial regeneration or lack factors necessary for the stimulation of the oligodendrocyte precursor cell population (Wolswijk, 1998). This concept led to the hypothesis that an efficient therapy for MS should not be limited to suppressing inflammation but should also favor remyelination. The remyelinating cells could originate from a variety of sources, including surviving oligodendrocytes native to the lesion, cells derived from these survivors, or the adjacent precursor cells. It has been shown that mature oligodendrocytes can be induced to dedifferentiate and proliferate by factors such as basic fibroblast growth factor (bFGF), suggesting a mechanism for regeneration of the oligodendroglial lineage following demyelinating disease (Grinspan et al., 1996; Grinspan et al., 1993).
Additional evidence for the beneficial effects of remyelination in demyelinating disorders such as MS is provided by the studies performed with glial growth factors as treatments in animal models of the disease. Glial growth factor 2 (neuregulin/GGF-2), a CNS growth factor known to promote oligodendrocyte proliferation and survival, was shown to delay disease onset, reduce clinical severity and decrease relapse frequency in the EAE murine model of MS (Marchionni et al., 1999). Neuregulin was shown to have a beneficial effect on mature oligodendrocyte survival and is produced by axons (Fernández et al., 2000).
Other growth factors, including platelet-derived growth factor (PDGF) and IGF-1, have been demonstrated to promote remyelination and have therapeutic effects in EAE models (reviewed in Dubois-Dalcq and Murray, 2000). The success achieved with the stimulation of remyelination, through inducing cells of the oligodendrocyte lineage to proliferate and/or differentiate, indicates that prospects for remyelination as a therapeutic strategy for MS are favorable. It would also be important to identify molecules that inhibit myelin synthesis, since these could lower the effectiveness of repair strategies such as oligodendroglial cell transplantation in MS.
The process of remyelination could work in concert with anti-inflammatory pathways to repair damage and protect axons from transection and death.
Oligodendrocytes may be induced to remyelinate axonal tracts in the CNS, thereby contributing to amelioration of the disease condition. Remyelination enhancement would counteract the previous destruction wrought by invasion of immune system cells into CNS tissue and their attack on myelin sheaths.
Several analyses of oligodendroglial differentiation and multiple sclerosis lesions have been performed using microarray visualization of differential gene expression (DGE, Scarlato et al., 2000; Whitney et al., 1999). These have utilized significantly different array technologies to assay varying sets of genes. Analysis of gene expression in both differentiating oligodendrocytes and multiple sclerosis lesions have indicated significant changes in the expression of myelin-specific genes. In addition, other genes were pinpointed as being differentially regulated, many of which were known to be involved in processes such as cell cycle control, cytoskeletal reorganization and membrane trafficking (Scarlato et al., 2000).
Osteopontin is a highly phosphorylated sialoprotein that is a prominent component of the mineralized extracellular matrices of bones and teeth. OPN is characterized by the presence of a polyaspartic acid sequence and sites of Ser/Thr phosphorylation that mediate hydroxyapatite binding, and a highly conserved RGD motif that mediates cell attachment/signaling. Expression of osteopontin in a variety of tissues indicates a multiplicity of functions that involve one or more of these conserved motifs. While the lack of a clear phenotype in OPN “knockout” mice has not established, a definitive role for osteopontin in any issue, recent studies have provided some novel and intriguing insights into the versatility of this protein in diverse biological events, including developmental processes, wound healing, immunological responses, tumorigenesis, bone resorption, and calcification. The ability of osteopontin to stimulate cell activity through multiple receptors linked to several interactive signaling pathways can account for much of the functional diversity (Sodek et al.).
Osteopontin has also been shown to be expressed in primary sensory neurons in the rat spinal and trigeminal nervous systems, both in the neuronal cell bodies and in the axons (Ichikawa et al., 2000).
Osteopontin mRNA is expressed in the adult brain as shown by in situ hybridization. Expression was found in neurons of the olfactory bulb and the brain stem, and in the latter it was found in functionally diverse areas including motor-related areas, sensory system and reticular formation (Shin et al., 1999).
Another study investigated the spatial and temporal expression of osteopontin mRNA following transient forebrain ischemia in rats. The transient induction of OPN mRNA after global ischemia occurred earlier in the striatum than in the hippocampus. It was pronounced in the dorsomedial striatum close to the lateral ventricle and in the CA1 subfield and the subiculum of the hippocampus before microglial cells became more reactive. It also could be detected in the dentate hilus, and to a marginal extent in the CA3 (Lee M Y, Shin S L, Choi Y S, Kim E J, Cha J H, Chun M H, Lee S B, Kim S Y, Neurosci Lett Aug. 20, 1999 271:2 81–4).
Osteopontin is also called Eta-1. WO 00/63241 relates to methods for modulating immune responses, in particular methods for modulating type 1 immune responses using modulators of Eta-1 (early T lymphocyte activation-1)/osteopontin. Osteopontin modulators are said to be useful for treatment of infections, immune disorders and diseases, autoimmune disorders, including MS, various immunodeficiencies, and cancer. All modulators of osteopontin disclosed in WO 00/63241, which are envisaged to be useful in autoimmune diseases, including MS, are inhibitors of osteopontin/Eta-1, as explained in detail in section V. “Clinical Applications of the Modulatory Methods of the Invention”, D “Autoimmune Diseases”, on page 51 to 53 of WO 00/63241.
Interferons are a subclass of cytokines that exhibit anti-inflammatory, antiviral and antiproliferative activity. On the basis of biochemical and immunological properties, the naturally-occurring human interferons are grouped into three classes: interferon alpha (leukocyte), interferon beta (fibroblast) and interferon gamma (immune). Alpha-interferon is currently approved in the United States and other countries for the treatment of hairy cell leukemia, venereal warts, Kaposi's Sarcoma (a cancer commonly afflicting patients suffering from Acquired Immune Deficiency Syndrome (AIDS)), and chronic non-A, non-B hepatitis.
Further, interferons (IFNs) are glycoproteins produced by the body in response to a viral infection. They inhibit the multiplication of viruses in protected cells. Consisting of a lower molecular weight protein, IFNs are remarkably non specific in their action, i.e. IFN induced by one virus is effective against a broad range of other viruses. They are however species-specific, i.e. IFN produced by one species will only stimulate antiviral activity in cells of the same or a closely related species. IFNs were the first group of cytokines to be exploited for their potential antitumour and antiviral activities.
The three major IFNs are referred to as IFN-α, IFN-β and IFN-γ. Such main kinds of IFNs were initially classified according to their cells of origin (leucocyte, fibroblast or T cell). However, it became clear that several types may be produced by one cell. Hence leucocyte IFN is now called IFN-α, fibroblast IFN is IFN-β and T cell IFN is IFN-γ. There is also a fourth type of IFN, lymphoblastoid IFN, produced in the “Namalwa” cell line (derived from Burkitt's lymphoma), which seems to produce a mixture of both leucocyte and fibroblast IFN.
The Interferon unit has been reported as a measure of IFN activity defined (somewhat arbitrarily) as the amount necessary to protect 50% of the cells against viral damage.
Every class of IFN contains several distinct types. IFN-β and IFN-γ are each the product of a single gene. The differences between individual types seem to be mainly due to variations in glycosylation.
IFNs-α are the most diverse group, containing about 15 types. There is a cluster of IFN-α genes on chromosome 9, containing at least 23 members, of which 15 are active and transcribed. Mature IFNs-α is not glycosylated.
IFNs-α and IFN-β are all the same length (165 or 166 amino acids) with similar biological activities. IFNs-γ are 146 amino acids in length, and resemble the α and β classes less closely. Only IFNs-γ can activate macrophages or induce the maturation of killer T cells. In effect, these new types of therapeutic agents can be called biologic response modifiers (BRMs), because they have an effect on the response of the organism to the tumour, affecting recognition via immunomodulation.
In particular, human fibroblast interferon (IFN-β) has antiviral activity and can also stimulate natural killer cells against neoplastic cells. It is a polypeptide of about 20,000 Da induced by viruses and double-stranded RNAs. From the nucleotide sequence of the gene for fibroblast interferon, cloned by recombinant DNA technology, Derynk et al. (Derynk R. et al, 1980) deduced the complete amino acid sequence of the protein. It is 166 amino acid long.
Shepard et al. (Shepard H. M. et al. 1981) described a mutation at base 842 (Cys→Tyr at position 141) that abolished its anti-viral activity, and a variant clone with a deletion of nucleotides 1119–1121.
Mark et al. (Mark D. F. et al, 1984) inserted an artificial mutation by replacing base 469 (T) with (A) causing an amino acid switch from Cys→Ser at position 17. The resulting IFN-β was reported to be as active as the ‘native’ IFN-β and stable during long-term storage (−70° C.).
Rebif® (recombinant human Interferon-β) is the latest development in interferon therapy for multiple sclerosis (MS) and represents a significant advance in treatment. Rebif® is interferon(IFN)-beta 1a, produced from mammalian cell lines and virtually identical to the naturally occurring human molecule.
The mechanisms by which IFNs exert their effects are not completely understood. However, in most cases they act by affecting the induction or transcription of certain genes, thus affecting the immune system. In vitro studies have shown that IFNs are capable of inducing or suppressing about 20 gene products
IFN-β may act by three major pathways in MS:                regulation of T-cell functions such as activation, proliferation and suppressor cell function;        modulation of the production of cytokines: down-regulation of proinflammatory cytokines and up-regulation of inhibitory, antiinflammatory cytokines;        regulation of T-cell migration and infiltration into the CNS via the BBB (blood brain barrier).        
The PRISMS study has established the efficacy of Interferon beta-1a given sub-cutaneously three times per week in the treatment of Relapsing-Remitting Multiple Sclerosis(RR-MS). This study showed that Interferon beta-1a can have a positive effect on the long-term course of MS by reducing the number and severity of relapses and reducing the burden of the disease and disease activity as measured by MRI. (Randomised, Double-Blind, Placebo-Controlled Study of Interferon beta-1a in Relapsing-remitting Multiple Sclerosis”. The Lancet 1998; 352 (Nov. 7, 1998): 1498–1504.)
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